Medical endoscopic probes have the ability to provide images from inside the patient's body. Considering the potential damage to a human body caused by the insertion of a foreign object, it is preferable for the probe to be as small as possible. Additionally, the ability to image within small conduits such as small vessels, small ducts, small needles, cracks, etc., requires a small probe size.
One useful medical probe employs spectrally encoded endoscopy (“SEE”), which is a miniature endoscopy technology that can conduct high-definition imaging through a sub-mm diameter probe. In a SEE probe, broadband light is diffracted by a grating at the tip of an optical fiber, producing a dispersed spectrum of the different wavelengths (colors) on the sample. Light returned from the sample is detected using a spectrometer; and each resolvable wavelength corresponds to reflectance from a different point on the sample. Thus, a SEE probe encodes light reflected from a given point in the sample by wavelength. The principle of the SEE technique and a SEE probe with a diameter of 0.5 mm, i.e., 500 μm have been described by D. Yelin et al., in a publication entitled “Three-dimensional miniature endoscopy”, Nature Vol. 443, 765-765 (2006). Another similar example is described by G. Tearney et al., in “Spectrally encoded miniature endoscopy”, Opt. Lett., 27(6): p. 412-414, 2002. Imaging with SEE can produce high-quality images in two- and three-dimensions.
Spectrally-encoded endoscopy utilizes the ability of the diffraction grating that deflects incident light to a diffraction angle according to wavelength. When the deflected light hits an object, light is scattered by the object. Detecting the scattered light intensity at each wavelength is equivalent to detecting the intensity from the corresponding diffraction angle. Thus, one-dimensional line image of the object is obtained. A two-dimensional image is obtained by rotating the SEE probe. A three-dimensional image can be obtained by rotating and translating (moving linearly) the SEE probe. Moreover, when incorporated into a sample arm of an interferometer, the SEE probe can also acquire depth information from a sample (e.g., tissue). Typically, as the grating deflects the light, the incident light is usually bent with respect to the optical axis of the probe. In this way, no light goes straight with respect to the optical axis. As no light goes straight, it is not possible with conventional spectrally-encoded endoscopy configuration to view in a forward direction.
Current trend of the spectrally-encoded endoscopy employs side-view type, with a few examples exhibiting forward viewing characteristics. The front-view type consists of multiple components including lenses, spacer elements, prisms and gratings, which makes the probe design complicated. Examples of such designs can be found, for example, in C. Pitris et al., Optical Express Vol. 11 120-124 (2003) and U.S. Pat. No. 8,145,018, both of which disclose a dual prism configuration where a grating is sandwiched between two prisms (a “grism”). This grism directs spectrally dispersed light in the directions including the optical axis of the fiber. The grism consists of multiple components (grating, prisms) which need proper alignment. The need of a grism to construct a forward-view probe increases the cost, complexity of fabrication and size of the probe. Publication WO2015/116951 discloses another forward view endoscope where the angled reflective side surface makes the light incidence angle on the grating such that at least one of the wavelengths propagates parallel to the optical axis of the lens. However, these known designs of forward view SEE probes have drawbacks. First, this design may not allow for use of the full available aperture. A smaller aperture means a decreased achievable resolution.
Second, both designs need a reflective surface in the spacer. This is not particularly easy to fabricate considering the miniature size of the spacer. In particular, the alignment of the spacer and the GRIN lens poses challenges during fabrication.
Further, the illumination fiber is off-axis to the GRIN lens, which introduces additional difficulties in fabrication as well as optical aberrations. In some designs, a reflective coating is needed at least for the second reflective surface, which will introduce light loss and scattering in the system. This coating is also needed for the first reflective surface unless a lower refractive index epoxy is used. A lower reflective index epoxy usually requires special curing conditions, which poses additional concerns for mass production.
Accordingly, it can be beneficial to address and/or overcome at least some of the deficiencies indicated herein above, and thus to provide a new SEE probe having forward direction view and/or omnidirectional view, and an apparatus to use such a probe, e.g., for imaging in a small optics. It is also beneficial to provide a SEE probe having a lower cost and/or less complexity compared to prior known probes.